Powder coating compositions of such conventional particle size are widely used in the industry. Some of the known drawbacks encountered with these materials are associated with the application characteristics of the powders. The ease with which the powder fluidizes and is transported through the application equipment affects the uniformity of film weight distribution across the substrate and as a consequence the amount of powder needed to achieve “on average” the desired film thickness. With these conventional powder coatings the first-time deposition of the fine particles (especially powder particles of 10 microns diameter or less) is inefficient leading to an accumulation of fine particles (fines) in the over-sprayed material. In many instances, the over-sprayed material is collected and recycled for re-use. In such systems, as the powder application process continues, the percentage of fine powder particles in the recycled powder increases and the cohesive behavior of the fines starts to influence the properties of the recycled powder. The major effect is that the powder loses fluidity and this causes increasing difficulties in transporting the powder through the recycle system and back to the spray gun.
Another well-known problem with using powder coating compositions of such conventional particle size distribution is the difficulty of applying thin film coatings of, say, 30 microns or less, for which there is an increasing demand in certain sectors of the powder coating market, while achieving a uniform opacity and an aesthetically pleasing appearance, particularly in a gloss white coating. Using compositions of conventional particle size distribution, the achievement of such results is possible only within a restricted range of powder chemistries, with the best performance generally being obtained with polyurethane powders using blocked isocyanates. To achieve coatings of less than 20 microns with a uniform opacity and an aesthetically pleasing appearance with compositions of conventional particle size distribution is very difficult if not impossible.
The problems encountered (“orange-peel” imperfections, etc.) are considered to be attributable to the relatively large size of the majority of the particles in powder coating compositions of conventional particle size distribution. In addition to the increasing demands within the powder coatings market itself, it has also been recognized that the inability of powder coatings reliably and routinely to achieve film thicknesses of 30 microns or less with aesthetically pleasing appearance is one of the factors that has inhibited further substitution of solvent-based “wet” paints by powder coatings.
It has been proposed that the problems of achieving satisfactory thin film powder coatings can in principle be alleviated with the use of powder coating compositions of a finer particle size. There are problems, however, in the fluidization, handling and application of relatively small particles, especially particles which are 10 microns in diameter or less. Such problems become more pronounced as the proportion of fine particles increases, and powder coating compositions have conventionally been manufactured so as to comprise not more than 10% by volume of particles which are 10 microns in diameter or less.
PCT Publication No. WO 94/11446 discloses powder coating compositions which incorporate, by dry-blending, various combinations of two or more additives, the preferred combination being aluminum oxide with aluminum hydroxide. By using combinations of dry-blended additives in accordance with PCT Publication No. WO 94/11446, it is possible to alleviate the problems of fluidization, handling and application of fine particles (especially those of 10 micron diameter or less) as outlined above and also to alleviate other problems attributable to differential and premature electrostatic charging of the powder particles. PCT Publication No. WO 94/11446 is directed to powder coating compositions of which at least 95% by volume has a particle size not exceeding 50 microns.
Although good results are achievable using the additive combinations disclosed in PCT Publication No. WO 94/11446, it has been found, inter alia, that the optimum blend of aluminum oxide/aluminum hydroxide, in terms of the fluidity, transport and handling characteristics of the powder coating composition, tends to detract from the utility of the composition in tribostatic application processes. Attempts to compensate for that effect by increasing the proportion of aluminum oxide in the additive combination tend to result in powder coating compositions having less good fluidity and transport properties.
European Patent No. EP-0300818 suggests that tribo-charging characteristics can be conferred on powder coating compositions of conventional particle size distribution by the use of an additive which comprises one or both of aluminum oxide and/or hydroxide, which has been sheared (by highspeed shearing or milling) such that the additive resulting therefrom is a fine, essentially aggregate-free powder which comprises at least 5% by weight of particles of maximum size 0.2 microns. According to European Patent No. EP-0300818, a preferred way of achieving the specified particle size requirement is to add to the aluminum oxide/hydroxide, prior to shearing, a proportion of a fine particle extender of maximum particle size 0.2 microns. The preferred extender disclosed in European Patent No. EP-0300818 is fumed silica, but it has been found that the inclusion of fumed silica tends to detract from the efficiency of an aluminum oxide/aluminum hydroxide additive (as proposed in PCT Publication No. WO 94/11446) in terms of fluidity and transport characteristics, so that the optimum benefit of the additive combination in terms of those characteristics is not achieved.
It would be desirable to find a powder coating composition which incorporates, by dry blending, a wax-coated silica in finely-divided form.